1. There are two kinds of walling; one like network, opus reticultatum, which all use now, and the old manner which is called opus incertum. Of these the reticulatum is more graceful, but it is likely to cause cracks because it has the beds and joints in every direction. The "uncertain" rough work, opus incertum, lying course above course and breaking joints, furnishes walling which is not pleasing but is tronger than the reticulatum.

2. Both kinds of walling are to be built with very minute stones; so that the walls, thoroughly saturated with mortar of lime and sand, may hold longer together. For since the stones are of a soft and open nature, they dry up the moisture by sucking it out of the mortar. But when the supply of lime and sand is abundant, the wall having more moisture will not quickly become perishable but holds together. When once, also, the moist power has been sucked out of the mortar, through the loose structure of the rubble, and the lime separates from the sand and is dissolved, the rubble also cannot cohere with them, but renders the walls ruinous with lapse of time.

3. This we may observe from some tombs which are built near the city, faced with marble or squared stone, and, in the interior, constructed with walling material pressed down. The mortar becomes perishable in time and is drawn out through the loose joints of the rubble. Hence the tombs collapse and disappear when the union of the joints is broken by settlement.

4. But if anyone does not wish to fall into this fault, let him keep the middle hollow behind the facings, and, on the inside, build walls two feet thick of red square stone or of baked brick or of lava, laid in proper courses, and let the facings be tied to these by iron clamps run in with lead. For thus the work is not built all of a heap but in order, and can last; because the beds and joints settling together and bound by ties do not thrust the work forward nor allow the facings bound in this way to give.

5. Therefore the walling of the Greeks is not to be made light of. For the do not employ walling of soft rubble with stucco facing, but when they depart from ashlar, they lay courses of lava or hard stone, and, as with brick buildings, they bind their joints in alternate courses, and so they produce strength firm enough to last. Well these are built in two kinds. Of these one is called isodomum, the other is called pseudoisodomum.

6. It is called isodomum when all the courses are built of an equal thickness; pseudoisodomum when the courses are unequal and unlike. Both are firm, for the reason especially that the rubble itself is of a thick and solid property, and cannot suck out the moisture from the mortar; the rubble preserves the mortar with its moisture for a long time; and the bed-joints of the stone, being laid flat and levelled, do not allow the mortar to sink down; but the stones being bonded in the unbroken thickness of the walls, keep the mortar together for a long time.

7. The second is that which they call enplecton, which our country people still use. In this the facings are dressed; the rest of the stones are laid with mortar in their natural state, and they bond them with alternate joints. But people nowadays, being eager for speedy building, attend only to the facing, setting the stones on end, and fill it up in the liddle with broken rubble and mortar. Thus three slices are raised in this walling, two of the facings, and a middle one of the filling in. Not so the Greeks who lay the stones level and put the headers and stretchers alternately. Thus they have not to fill in the middle, but with their through facing stones they render solid the unbroken and single thickness of the walls. In addition to the rest, they insert special stones facing on either front of unbroken thickness. These they call diatonos (through-stones), and they, by bonding, especially strengteh the solidity of the wall.

8. Therefore if anyone will from these commentaries observe and select a style of walling, he will be able to take account of durability. For those which are of soft rubble with a thin and pleasing facing cannot fall to give way with lapse of time. Therefore when arbitrators are taken for party-walls, they do not value them at the price at which they were made, but when from the accounts they find the tenders for them, they deduct as price of the passing of each year the 80th part, and so - in that from the remaining sum repayment is made for these walls - they pronounce the opinion that the walls cannot last more than 80 years.

9. There is no deduction made from the value of brick walls provided that they remain plumb; but they are always valued at as much as they were built for. Therefore in some cities we may see both public works and private houses and even palaces built of brick: and first, the wall at Athens which looks to mount Hymettus and Pentelicus; also at Patrae, brick cellae in the temple of Jupiter and Hercules, while round the temple there are entablatures and columns of stone; in Italy at Arezzo there is an old brick wall excellently built. At Tralles there is a palace built for the Attalid kings, which now is always given for a houose to him who is the Priest of the City: also at Lacedaemon the bricks were cut through from certain walls, the paintings were removed and enclosed in wooden frames, and brought into the Comitium as an ornament for the aedileship of Varro and Murena.

10. There is the palace of Croesus, which the people of Sardis dedicated to their fellow-citizens for repose in the leisure of their age, as an Almshouse for the College of the Elders. At Halicarnassus also, although the palace of the mighty king Mausolus had all parts finished with Proconnesian marble, it has walls built of brick. And these to this day maintain a striking firmness, being so finished with plaster work that they seem to have the translucency of glass. Nor was it for lack of means that the king did this. For he was enriched by enormous revenues because he ruled over all Caria.

11. We may thus consider his shrewdness and skill in providing buildings. For although he was born at Melisso, he observed at Halicarnassus a place naturally fortified, a suitable market, and a useful harbour, and he there established his palaca. Now that place is like the curvature of a theatre. The forum is placed at the lowest level along the harbour. But about the middle of the natural amphitheatre and, as it were, in a cross gangway, a street is constructed of ample width, in the middle of which the Mausoleum is built of such splendid workmanship that it is named among the Seven Sights of the World. In the middle of the top of the citadel is a temple of Mars having a statue of a colossus with marble extremities made by the famous hand of Leochares, others by Timotheus. On the right wing at the top is a temple of Venus and Mercury against Salmacis' fountain itself.

12. This fountain, however, by a mistaken opinion, is thought to afflict with an aphrodisiac disease those who drink from it. And why this opinion has wandered over the world through mistaken rumour it will not be inconvenient to set forth. For this cannot be because, as it is said, people are made effeminate and shameless by that water; the virtue of the spring is clearness and its flavour is excellent. Now when Melas and Arevanias led thither a joint colony from Argos and Troezen, they cast out the barbarians, Carians and Leleges. But these being driven to the hills, gathered together and made raids, and by brigandage they devastated the Greeks cruelly. But afterwards one of the colonists, for the sake of profit, fitted up an inn with complete supplies, near the spring, on account of the goodness of the water, and running the inn, he began to attract the barbarians. So coming down one by one, and mixing with society, they changed of their own accord from their rough and wild habits to Greek customs and affability. Therefore this water obtained such a reputation, not by the plague of an immodest disease, but through the softening of savage breasts by the delights of civilisation.

13. Since now I am brought to the description of these walls, it remains to outline it completely as they are. For just as on the right side there are the temple of Venus and the spring above described, so on the left wing is the royal palace which King Mausolos had built to his own plan. From it there is seen on the right side the forum and harbour and the whole circuit of the walls; under the left there is a secret harbour lying hid under high ground, in such a way that no one can see or know what is going on in it, so that the king from his own palace could see what is necessary for his sailors and soldiers, without anyone else knowing.

14. Therefore when, after the death of Mausolos, his wife Artemisia began to reign, the Rhodians were indignant that a woman should rule over the cities throughout Caria, and equipped a fleet they set out to seize the kingdom. It was reported to Artemisia. She hid the fleet in the harbour, concealing the rowers and the marines she had got together, and ordered the rest of the citizens to man the walls. Now when the Rhodians had landed, with a fleet well equipped, in the greater harbour, she commanded the citizens to greet them from the walls and to promise to surrender the town. These left their ships unmanned and penetrated within the wall. Artemisia, using an artificial outlet into the sea, suddenly led out her fleet from the lesser harbour and thus sailed into the greater. She then landed her soldiers and took the empty Rhodian fleet away to sea. So the Rhodians, having no place of retreat, were surrounded and killed in the forum itself.

15. So Artemisia, placing her own troops and rowers in the ships of the Rhodians sailed for Rhodes. But when the Rhodians saw their own ships wreathed with laurel, they thought their fellow citizen returned victorious and let the enemy in. The Artemisia took Rhodes, killed the leading citizens, and set up a trophy of her victory in the city of Rhodes, the other in her own likeness. She had the latter figured as settinga brand upon the city of Rhodes. But afterwards the Rhodians, being restrained by a religious scruple because it is forbidden for trphies once dedicated to be removed, erected a building round the spot and protected it with a Greek outpost to prevent anyone seeing, and ordered this to be called 'unapproachable'.

16. Since, therefore, kings of very great power have not disdained walls built of brick (in cases where wealth gained by taxation and plunder allowed the use not only of rubble or squared stone, but even of marble), I do not think that buildings which are made of brick walls are to be disregarded so long as they are duly roofed. But why this fashion ought not to be followed out by the Roman people in the city I will set forth, and will not omit the causes and reasons of this.

17. Public statutes do not allow a thickness of more than a foot and a half to be used for party walls. But other walls also are put up of the same thickness lest the space be too much narrowed. Now brick walls of a foot and a half - not being two or three bricks thick - cannot sustain more than one story. Yet with the greatness of the city and the unlimited crowding of citizens, it is necessary to provide very numerous dwellings. Therefore since a level site could not receive such a multitude to dwell in the city, circumstances themselves have compelled the resort to raising the height of buildings. And so by means of stone pillars, walls of burnt brick, party walls of rubble, towers have been raised, and these being joined together by frequent board floors produce upper stories with fine views over the city to the utmost advantage. Therefore walls are raised to a great height through various stories, and the Roman people has excellent dwellings without hindrance.

18. Now, therefore, the reason is explained why, because of the limited space in the city, they do not allow walls to be of sun-dried bricks. When it shall be necessary to use them, outside the city, such walls will be sound and durable after the following manner. At the top of the walls let walling of burnt brick be put beneath the tiles, and let it have a projecting cornice. So the faults which usually happen here can be avoided. For when tiles in the roof are broken or thrown down by the wind (where rain-water could pass through from showers), the burnt brick shield will not allow to brickwork to be damaged; but the projection of the cornices will throw the drippings outside the facing line, and in that way will keep intact the structure of brick walls.

19. But whether the baked brick itself is very good or faulty for building, no one can judge its strength off hand, because only when it is laid as a coping is it tested by weathering and lapse of time. For brickwork that is not made of good clay or is too little baked shows its faults on the work when wheatered by ice or hoar-frost. Therefore the brickwork which cannot stand the strain in the coping courses cannot be strong enough in the walling to carry loads. Wherefore the coping courses are specially built from old tiles, and the walls will be strong enough.

20. I could wish that walls of wattlework had not been invented. For however advantageous they are in speed of erection and for increase of space, to that extent are they a public misfortune, because they are like torches ready for kindling. Therefore it seems better to be at greater expense by the cost of burnt brick than to be in danger by the convenience of wattlework walls: for these also make cracks in the plaster covering owing to the arrangement of the uprights and cross-pieces. For when the plaster is applied, they take up the moisture and swell, then when they dry they contract, and so they are rendered thin, and break the solidity of the plaster. But since haste, or lack of means, or partitions made over an open space, somtimes require this construction, we must proceed as follows. Let the foundations be lais high up, so that it is untouched by the rough stones of the pavement; for when they are fixed in these, they become rotten in time; then they settle, and falling forward they break through the surface of the plaster.
With respect to walls and the use of material after its kinds, I have explained their excellences and faults as I have been able. Now with respect to floors and the material from which they are provided, so that they may not be weakened by lapse of time, I will explain as nature shows.

COMMENT

The general structure of this chapter once again shows the purposes of Vitruvius when he wrote his 'De Architectura'. It starts (sentences 1 - 9a) with an explanation of the different kinds of walling that were used in Roman architecture, their advantages and disadvantages, and a comparison to Greek building practices.
In sentence 9b he gives a lot of examples of cities where these building techniques were used in city walls. This enumeration of examples is interwoven with a few anecdotical remarks. And then he comes to Halicarnassos (sentences 10 - 15). This is the occasion to write a great digression about the layout of this city and of the history of a conflict with the Rhodians in which Artemisia, the wife of king Mausolos, played a prominent role. But before he comes to this history we hear about a certain fountain (sentence 12), the water of which should provoke veneral diseases. One can wonder what all this has to do with building technique and if the knowledge of these histories is advantageous to the architect and builder who has to construct a wall.
Te answer is simple: the 'De Architectura' was not meant for a public of experts but in the first place for a general public to which Vitruvius wanted to justify architecture as an art. It is obvious that such a book must contain the quintessens of architecture, but, on the other hand, to keep the attention of the reader and to make appeal on his curiosity it was also necessary to insert a number of anecdotes, mythological and historical digressions.
That this digression stands completely apart from his explanation about walling is proven by the ease with which in sentence 16 he comes back to his main discourse: he needs only three lines to come back to the walling techniques to which the rest of the chapter (sentences 16 - 20) is dedicated.

When we make abstraction from this digression we can see how this chapter falls apart in two sections. The first is about the kinds of Roman walling as compared to Greek walling (sentences 1 - 8), the second is about the properties of unbaked bricks in walls (sentences 16 - 19) with a little note on wattlework.

Vitruvius mades a distinction between opus incertum and opus reticulatum. Both kinds of walling were facings of a concrete or rubble core. Opus incertum consists of irregular blocks and gives a rather disorderly impression while opus reticulatum is made of regular square stones. Opus incertum appears in Roman architecture from the beginning of the 2nd century B.C. but it is quickly superseded by the more beautiful and more regular opus reticulatum.

Opus incertum

Opus reticulatum

Vitruvius compares this method to the Greek building technique which consist mainly of the superposition of horizontal layers of squared natural stones. According to the thickness of these layers he discerns opus isodomum in which all the layers are of a even thickness and opus pseudoisodomum in which the thickness of the layers can differ considerably. In contrast to Roman building practice the concept of this construction was a massive wall which was completely built up of stones. The concrete filling was unknown to the Greeks.

Opus isodomum

Opus pseudoisidomum

A third technique, which can roughly be compared to the Roman building practice, is the so called opus emplecton. This type of walling consists of two external facings composed of stones only worked on the visible site. The interior of the wall is filled with rubble. This type is known in Greek architecture from the end of the 7th century B.C.

In the illustrations below I give examples of a section of a wall in opus isodomum and pseudoisodomum as compared to opus emplecton.

Section of a walla) opus isodomumb) opus pseudoisodomum

Section of a wallin opus emplecton

The second section (sentences 16 - 20) is about the properties of walls in sun-dried bricks. It is obvious that water and rain as cause of erosion is the greatest enemy of this material. That's why Vitruvius gives the advice to cover sun-dried walls with a few layers of baked bricks and to construct a roof with a great outward projection. On the other hand Sun-dried bricks are not strong enough to support the great loads of a multi-storied building. Because of the lack of space Roman law forbade partition walls of more than 1,5 foot thick. On the other hand, also because of the lack of space, it was necessary to build dwellings with multiple stories. A wall with a thickness of 1 foot (or the length of 1 brick) could only support a roof. When one wished to add a second story it was necessary to construct a wall of 2 feet (or the length of 2 bricks), for a third story the thickness of an additional brick was needed, etc. But speculators wishing to get the highest possible profit, constructed always higher buildings with always thinner partition walls. This practice increased the danger of collapse of these buildings, and, indeed, collapses were a recurrent problem in antique Rome. In order to counter this phenomemen laws were voted from the middle of the 2nd century B.C. to limit the height of buildings. Infractions on this legislation were legion. Finally Augustus issued his Lex Iulia de modo aedificiorum urbis in which the height of a building, as seen from the street, was limited to 70 feet (20,65 metres) (the law said nothing about the parts of the building that were not adjacent to the public way). This law applied to all new buildings. Already existant buildings must be reduced to 70 feet. From this it may be clear that in late republican and early imperial Rome many tenement houses were higher and that passers by were endangered by a possible collapse of one of these constructions.